Mixed Layer Deepening and Internal Wave Generation under Sea Ice in Free Drift

Abstract The Arctic Ocean stores enough heat to melt the entire sea ice pack, but that heat is isolated from the surface due to strong salinity stratification. Processes like mixed layer deepening and internal wave breaking can erode this stratification and mix heat to the surface. While the deepening of a mixed layer of depth h into an ocean with constant buoyancy frequency N 0 under constant surface forcing is well understood for times (fast deepening; h ∝ t α with α = 0.5), where f is the Coriolis parameter, there is limited consensus on α for t ≳ f −1 (slow deepening). Since f −1 is smallest at polar latitudes, this unsettled regime may be relevant for the Arctic. Moreover, how the amount of energy fluxed into the internal wave field during fast and slow deepening may vary is largely unexplored. Using large-eddy simulations of the ice–ocean boundary layer, we investigate both the rate of slow deepening and the associated internal wave field. We find that sea ice in free drift imposes a nearly constant surface momentum flux on the ocean, even during inertial oscillations. We estimate α = 0.21 for slow deepening, which differs from fast deepening due to the surface power input tending toward a constant over time. Additionally, the resulting internal wave field is weaker during slow deepening because the internal wave energy flux decays as ∼ h −2 . Our results clarify the dynamics of slow deepening and its role in internal wave generation, both in general and in an Arctic context. Significance Statement The purpose of this study is to gain deeper insight into how energy from the atmosphere input at the ice–ocean boundary layer contributes to mixing the upper Arctic Ocean when the dynamics are affected by Earth’s rotation. Using numerical simulations and analytical arguments, we show that Earth’s rotation complicates this energy transfer by modifying the amount of power the ocean receives from the atmosphere. This leads to two distinct dynamical regimes that result in differing degrees of upper-ocean mixing. These results can be used to improve our understanding of how atmospheric forcing can draw stored heat in the Arctic Ocean to the surface, a topic that remains relevant for predicting the future of the Arctic sea ice cover.